This invention relates to a lithium secondary battery able to be used to drive the motor of an electric vehicle, and more particularly to a lithium secondary battery which can reduce internal resistance and attain good charging-discharging characteristics, as well as an high output and a huge current, by setting the shape parameters of an internal electrode body under predetermined conditions.
In recent years, with the heightening of the movement to protect the environment, and the eagerness to regulate emission of exhaust gasses, including carbon dioxide as well as other harmful matters, in the automobile industry, the replacement of automobiles using fossil fuels, such as a vehicle driven by gasoline, the movement to promote an introduction of an electric vehicle (EV), and a hybrid electric vehicle (HEV) has become active.
As a battery able to drive motors of such EV as well as HEV, a lithium secondary battery with a large energy density is expected to be promising battery. However a lithium secondary battery is required to attain characteristics such as large capacity, as well as high output so as to attain predetermined acceleration performance, slope-climbing performance, and continuous running performance. For example, when an HEV is in a mode to require output power at the time of acceleration, therefore, a high output is required for a battery to drive a motor. Here, the voltage per battery depends on materials configuring the battery. In the case of a lithium secondary battery, the voltage is approximately 4.2 V at highest. Moreover, a huge output means a huge current to flow.
On the other hand, when a plurality of batteries are connected in series for use in securing the required voltage for driving a motor, the same quantity of current will flow through each battery. Actually, in an HEV, it can frequently occur that a current not less than 100 A flows. Accordingly, it is important to reduce the internal resistance of a battery as much as possible so that such high output characteristics, as well as huge current characteristics, are realized.
Here, the internal resistance of a battery is determined by the resistance value of configuring members of the battery. It is extremely important that the resistance of an internal electrode body (charge-discharge unit) to be regarded as a central part of a battery should be reduced.
However, lithium secondary batteries currently in practical use for electric appliances, do not have large battery capacities and there are few cases that large currents have been required in their use, therefore no detailed studies on the resistance of the internal electrode body in the battery have been conducted to date.
For example, Japanese Patent No. 2701347 specification discloses a lithium secondary battery comprising a band-shaped positive electrode, and a band-shaped negative electrode, both electrodes being wound around each other in order to configure a wound body, wherein the positive active material layer configuring the band-shaped positive electrode and the negative active material layer configuring the band-shaped negative electrode have a predetermined thickness respectively, and the ratio of respective thickness falls within a predetermined range.
The lithium secondary battery disclosed in Japanese Patent No. 2701347 specification is one in which the elements configuring the internal electrode body, are regulated nevertheless the resistance of the internal electrode body has not been studied. That is, in spite of the fact that the thickness of the electrode active material layer is a factor affecting resistance of the internal electrode body, this point has not been discussed. In addition, no consideration has been given to the influence of the resistance of electrode substrate (current collector (metal foil)) itself, which is indispensable in configuring a band-shaped electrode as well as of the band-shaped electrode itself to the resistance of the internal electrode body.
In addition, the lithium secondary battery disclosed in Japanese Patent No. 2701347 specification has been made mainly to be used as a power source for an electronic appliance, aiming at fulfilling the requirement for larger capacity, that is, mainly aiming at improvement in energy density. In addition, it aims to prevent the occurrence of cracks in an electrode, and therefore, in Japanese Patent No. 2701347 specification no descriptions indicating that higher output has been the purpose thereof are found.
Thus, since development of a compact battery has been aimed mainly at achieving higher energy density, it is of concern how to provide a lithium secondary battery the output of which serious consideration has been given. Also, the relationship between thickness of electrode substrate of each positive and negative electrode and thickness of electrode active material layer would affect the resistance of an internal electric body was not well understood. In addition, the density of winding, that is, the number of turns of the positive electrode or the negative electrode per unit length along the direction of diameter of the internal electrode body, was similarly not well understood.
Moreover, Japanese Patent No. 2701347 specification describes that it is preferable that lithium cobalt oxide (LiCoO.sub.2), with a large lithium capacity, as the positive active material be used since the disclose lithium secondary battery is formed with the main purpose being improvement of energy density, as described above.
However, in the case where the main purpose is reduction in the resistance of an internal electrode body, as well as higher output, the quantity of lithium in a positive active material may be lessened to fall within a range which can secure the predetermined quantity. In this case, the use of positive active material in making resistance of the positive active material layer smaller is regarded as more important.
Incidentally, also as for a lithium secondary battery with a large capacity mainly aiming at the above-described higher output such as to be used in EV, etc., it goes without saying that weight should be preferably lighter if the battery capacity is the same. In addition, if the volume of a battery is smaller, less mounting space is required in a vehicle, preferably resulting in larger freedom in space designing. Moreover, it is costly disadvantageous as well to use many materials without necessity.